The Echinoid Directory

Key to genera of Echinoida (3)

11a.Test with ambitus at midheight or below; domal to subconical in profile.

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11b.Test with ambitus above midheight so that the test is inverted conical in profile with flat top and base.

Echinostrephus

12a.Pore-pairs in arcs either rather vertical, or oblique so as to form a single adradial band.

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12b.Pore-pairs strongly offset horizontally so as to form distinct vertical series.

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13a.Primary tubercles indistinct from secondary tubercles on aboral interambulacral plates.

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13b.Primary interambulacral tubercles clearly distinct from secondaries, especially aborally.

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14a.Primary ambulacral tubercle on every plate.

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14b.Primary ambulacral tubercle on every second or third plate.

Echinus

15a.Ambulacral plates with 2 subequal tubercles

Isechinus

15b.Ambulacral plates with one large primary tubercle

Psammechinus

16a.Interambulacral primary tubercles set on vertical ridge

Stirechinus

16b.Interambulacral plates without vertical ridge connecting primary tubercles

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